Fix #4936: Forbid (or warn on) redundant/illegal object flags

compiler/src/dotty/tools/dotc/core/NameOps.scala

@@ -10,7 +10,7 @@ import Definitions._
 
 object NameOps {
 
-  final object compactify {
+  object compactify {
     lazy val md5: MessageDigest = MessageDigest.getInstance("MD5")
 
     /** COMPACTIFY

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2415,6 +2415,16 @@ object Parsers {
 
     def objectDefRest(start: Offset, mods: Modifiers, name: TermName): ModuleDef = {
       val template = templateOpt(emptyConstructor)
+
+      def flagPos(flag: FlagSet) = mods.mods.find(_.flags == flag).get.pos
+      if (mods is Abstract)
+        syntaxError(hl"""${Abstract} modifier cannot be used for objects""", flagPos(Abstract))
+      if (mods is Sealed)
+        syntaxError(hl"""${Sealed} modifier is redundant for objects""", flagPos(Sealed))
+      // Maybe this should be an error; see https://github.com/scala/bug/issues/11094.
+      if (mods is Final)
+        warning(hl"""${Final} modifier is redundant for objects""", source atPos flagPos(Final))
+
       finalizeDef(ModuleDef(name, template), mods, start)
     }
 

compiler/src/dotty/tools/dotc/printing/Formatting.scala

@@ -80,9 +80,8 @@ object Formatting {
           hl.show
         case hb: HighlightBuffer =>
           hb.toString
-        case str: String =>
-          SyntaxHighlighting.highlight(str)
-        case _ => super.showArg(arg)
+        case _ =>
+          SyntaxHighlighting.highlight(super.showArg(arg))
       }
   }
 

compiler/test/dotty/tools/vulpix/Status.scala

@@ -4,4 +4,4 @@ package vulpix
 sealed trait Status
 final case class Success(output: String) extends Status
 final case class Failure(output: String) extends Status
-final case object Timeout extends Status
+case object Timeout extends Status

doc-tool/src/dotty/tools/dottydoc/model/references.scala

@@ -11,7 +11,7 @@ object references {
   final case class BoundsReference(low: Reference, high: Reference) extends Reference
   final case class NamedReference(title: String, ref: Reference, isByName: Boolean = false, isRepeated: Boolean = false) extends Reference
   final case class ConstantReference(title: String) extends Reference
-  final case object EmptyReference extends Reference
+  case object EmptyReference extends Reference
 
   /** Use MaterializableLink for entities that need be picklable */
   sealed trait MaterializableLink { def title: String }

semanticdb/src/dotty/semanticdb/Scala.scala

@@ -148,7 +148,7 @@ object Scala {
     }
   }
   object Descriptor {
-    final case object None extends Descriptor { def value: String = "" }
+    case object None extends Descriptor { def value: String = "" }
     final case class Term(value: String) extends Descriptor
     final case class Method(value: String, disambiguator: String) extends Descriptor
     final case class Type(value: String) extends Descriptor

tests/neg-custom-args/fatal-warnings/i4936.scala

@@ -0,0 +1 @@
+final object Foo // error

tests/neg/i3471.scala

@@ -0,0 +1 @@
+sealed object Fun // error

tests/neg/i4936.scala

@@ -0,0 +1,4 @@
+abstract object Foo // error
+sealed final abstract case object Bar // error // error
+
+abstract override object Baz // error

tests/patmat/i3645.scala

@@ -13,8 +13,8 @@ object App {
     }
 
     sealed abstract class K[A]
-    final case object KAge extends K[Age]
-    final case object KInt extends K[Int]
+    case object KAge extends K[Age]
+    case object KInt extends K[Int]
 
     val kint: K[Age] = Age.subst[K](KInt)
     def get(k: K[Age]): String = k match {

tests/patmat/i3645b.scala

@@ -13,9 +13,9 @@ object App {
     type Bar = Foo
 
     sealed abstract class K[A]
-    final case object K1 extends K[Int]
-    final case object K2 extends K[Foo]
-    final case object K3 extends K[Bar]
+    case object K1 extends K[Int]
+    case object K2 extends K[Foo]
+    case object K3 extends K[Bar]
 
     val foo: K[Int] = Foo.subst[K](K2)
     def get(k: K[Int]): Unit = k match {

tests/patmat/i3645c.scala

@@ -13,9 +13,9 @@ object App {
     type Bar = Foo
 
     sealed abstract class K[+A]
-    final case object K1 extends K[Int]
-    final case object K2 extends K[Foo]
-    final case object K3 extends K[Bar]
+    case object K1 extends K[Int]
+    case object K2 extends K[Foo]
+    case object K3 extends K[Bar]
 
     val foo: K[Int] = Foo.subst[K](K2)
     def get(k: K[Int]): Unit = k match {

tests/patmat/i3645d.scala

@@ -13,9 +13,9 @@ object App {
     type Bar = Foo
 
     sealed abstract class K[-A]
-    final case object K1 extends K[Int]
-    final case object K2 extends K[Foo]
-    final case object K3 extends K[Bar]
+    case object K1 extends K[Int]
+    case object K2 extends K[Foo]
+    case object K3 extends K[Bar]
 
     val foo: K[Int] = Foo.subst[K](K2)
     def get(k: K[Int]): Unit = k match {

tests/patmat/i3645e.scala

@@ -22,8 +22,8 @@ object App {
     type Foo = Module.Foo.Type
 
     sealed abstract class K[F]
-    final case object K1 extends K[Int]
-    final case object K2 extends K[Foo]
+    case object K1 extends K[Int]
+    case object K2 extends K[Foo]
 
     val kv: K[Foo] = Module.Foo.subst[K](K1)
     def test(k: K[Foo]): Unit = k match {

tests/patmat/i3645f.scala

@@ -23,8 +23,8 @@ object App {
     type Foo = Module.Foo.Type
 
     sealed abstract class K[F]
-    final case object K1 extends K[Int]
-    final case object K2 extends K[Foo]
+    case object K1 extends K[Int]
+    case object K2 extends K[Foo]
 
     val kv: K[Foo] = Module.Foo.subst[K](K1)
     def test(k: K[Foo]): Unit = k match {

tests/patmat/i3645g.scala

@@ -22,8 +22,8 @@ object App {
     type Foo = Module.Foo.Type
 
     sealed abstract class K[F]
-    final case object K1 extends K[Int]
-    final case object K2 extends K[Foo]
+    case object K1 extends K[Int]
+    case object K2 extends K[Foo]
 
     val kv: K[Foo] = Module.Foo.subst[K](K1)
     def test(k: K[Foo]): Unit = k match {

tests/patmat/t7353.scala

@@ -1,8 +1,8 @@
 sealed trait EthernetType
 
 object EthernetType {
-  final case object Gigabit extends EthernetType
-  final case object FastEthernet extends EthernetType
+  case object Gigabit extends EthernetType
+  case object FastEthernet extends EthernetType
 
   final def toInt(t: EthernetType) = t match {
     case Gigabit => 1

tests/pos-kind-polymorphism/anykind.scala

@@ -93,7 +93,7 @@ object Test {
   sealed trait HList
   final case class ::[+H, +T <: HList](head : H, tail : T) extends HList
   sealed trait HNil extends HList
-  final case object HNil extends HNil
+  case object HNil extends HNil
 
   object New {
     // The Kind Polymorphic List

tests/pos/byname-implicits-13.scala

@@ -13,7 +13,7 @@ case class Cat(name: String, friend: Either[Cat, Dog])
 sealed trait HList extends Product with Serializable
 final case class ::[+H, +T <: HList](head: H, tail: T) extends HList
 sealed trait HNil extends HList
-final case object HNil extends HNil
+case object HNil extends HNil
 
 sealed trait Coproduct extends Product with Serializable
 sealed trait :+:[+H, +T <: Coproduct] extends Coproduct

tests/pos/byname-implicits-14.scala

@@ -13,7 +13,7 @@ case class Cat(name: String, friend: Either[Cat, Dog])
 sealed trait HList extends Product with Serializable
 final case class ::[+H, +T <: HList](head: H, tail: T) extends HList
 sealed trait HNil extends HList
-final case object HNil extends HNil
+case object HNil extends HNil
 
 sealed trait Coproduct extends Product with Serializable
 sealed trait :+:[+H, +T <: Coproduct] extends Coproduct

tests/pos/i3647.scala

@@ -10,8 +10,8 @@ object App {
       type T = String
       type Bar[A] = J[A]
       sealed abstract class J[A]
-      final case object JName extends J[T]
-      final case object JInt extends J[Int]
+      case object JName extends J[T]
+      case object JInt extends J[Int]
 
       def get(k: J[T]): String = k match {
         case JName => "Age"

tests/run/erased-frameless.scala

@@ -5,7 +5,7 @@ import scala.annotation.implicitNotFound
 
 sealed trait HList
 sealed trait HNil extends HList
-final case object HNil extends HNil
+case object HNil extends HNil
 final case class ::[H, T <: HList](h: H, t: T) extends HList
 
 /** Generic representation os type T as a labelled sum of product. */

tests/run/hmap-covariant.scala

@@ -1,6 +1,6 @@
 trait Tuple
 case class TCons[+H, +T <: Tuple](h: H, t: T) extends Tuple
-final case object TNil extends Tuple
+case object TNil extends Tuple
 
 // Type level natural numbers -------------------------------------------------
 

tests/run/hmap.scala

@@ -1,6 +1,6 @@
 trait Tuple
 case class TCons[H, T <: Tuple](h: H, t: T) extends Tuple
-final case object TNil extends Tuple
+case object TNil extends Tuple
 
 // Type level natural numbers -------------------------------------------------
 

tests/run/phantom-OnHList.scala

@@ -24,7 +24,7 @@ sealed trait HNil extends HList
 
 // HList values -----------------------------------------------------------------------------------
 
-final case object HNil extends HNil {
+case object HNil extends HNil {
   val underlying: Array[Any] = Array.empty[Any]
   override def toString(): String = "()"
 }